The objectives of this study are to determine the morphological characteristics, evaluate the roles, and define the interactions of stromal cells of the hematopoietic microenvironment which influence or regulate hematopoietic stem cells and progenitor cells. Monolayers of stromal elements from various hematopoietic organs are being established as primary cultures. Enriched and/or cloned populations of stromal cells are then derived from these heterogenous primary monolayers. Cells obtained in primary, enriched and cloned cultures are being studied utilizing both in vivo and in vitro techniques. Monolayers are tested for their ability to support CFU-S & CFU-C in the Dexter liquid culture system. Conditioned media is analyzed routinely for the presence of humoral regulators of hematopoiesis such as GM-CSF using both bioassay and radioimmunoassay techniques. Cells harvested from monolayers are transplanted back to an in vivo site in normal or hematopoietically stressed hosts to assess their ability to participate in the reformation of a functional hematopoietic microenvironment. In future studies, mixtures of cloned or enriched stromal cells will also be employed in both the in vivo and in vitro systems to address the problem of multiple cell interactions in hematopoiesis. Stromal cell grafts as well as monolayers are evaluated using routine histochemical and histophysiological techniques at both the LM and EM level. Chromosome markers will be used to investigate the origin of both the stromal cell and hematopoietic cell populations found in the graft. Finally, stromal cell populations will be cultured in matrix systems to study their ability to form a functional three-dimensional hematopoietic microenvironment in vitro. This study will provide information that is relevant not only to our understanding of mammalian cell biology and in particular cellular hematology, but also potentially useful in clinical hematology.